1. Field of the Invention
The invention relates to dielectric films, more particularly nanoporous dielectric films, and to a process for their manufacture. The nanoporous dielectric films provide low dielectric constants. Such films are useful in the production of integrated circuits.
2. Description of the Prior Art
In the production of advanced integrated circuits that have minimum feature sizes of 0.25 micrometers and below, problems of interconnect RC delay, power consumption and crosstalk become significant. The use of low dielectric constant (K) materials for interlevel dielectric and intermetal dielectric applications partially mitigate these problems. However, each of the material candidates which are under consideration by the industry, having dielectric constants significantly lower than the currently employed dense silica, suffer from disadvantages. Most low dielectric constant materials developments use spin-on-glasses and fluorinated plasma chemical vapor deposition SiO2 with K of greater than 3. Some organic and inorganic polymers have dielectric constants in the range of about 2.2 to 3.5, however, these have the problems of low thermal stability, poor mechanical properties including low glass transition temperature, sample outgassing, and long term reliability questions.
Another approach has been to employ nanoporous silica which can have dielectric constants in the range of about 1 to 3. Porous silica is attractive because it employs similar precursors, e.g. tetraethoxysilane (TEOS) as is presently used for spin-on glass (SOG""s) and CVD SiO2, and due to the ability to carefully control pore size and pore distribution. In addition to having low dielectric constants, nanoporous silica offers other advantages for microelectronics including thermal stability up to 900xc2x0 C.; small pore size ( less than  less than microelectronics features), use of materials, namely silica and its precursors, that are widely used in the semiconductor industry; the ability to tune dielectric constant over a wide range; and deposition using similar tools as employed for conventional spin-on glass processing. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows one method for producing a nanoporous silica film with uniform density throughout the film thickness.
Higher porosity materials not only leads to a lower dielectric constant than dense materials, but such also allow additional components and processing steps to be introduced. Materials issues include, the need for having all pores significantly smaller than circuit feature sizes; the strength decrease associated with porosity; and the role of surface chemistry on dielectric constant, loss and environmental stability. Density, or its inverse, porosity, is the key nanoporous silica parameter controlling property of importance for dielectrics. Properties may be varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to dense silica with a porosity of 0%. As density increases, dielectric constant, hardness and mechanical strength increase but the pore size decreases.
Nanoporous silica films are fabricated by using a mixture of a solvent and a silica precursor which is deposited on a wafer by conventional methods such as spin-coating, dip-coating, etc. The precursor must polymerize after deposition and be sufficiently strong such that it does not shrink during drying. Film thickness and density/dielectric constant can be controlled independently by using a mixture of two solvents with different volatility. The more volatile solvent evaporates during and immediately after precursor deposition, while the less volatile solvent remains as the pore fluid. The silica precursor, typically a partially hydrolyzed and condensed product of TEOS, is polymerized by chemical and/or thermal means until it forms a gel layer. Using this approach, a nanoporous silica film is obtained with uniform density throughout the film thickness.
Normally in sol-gel processing to produce porous silica, a liquid catalyst such as an acid or base is added to the silica precursor/solvent mixture in order to initiate polymerization. This catalyst addition is often accompanied by the addition of water which is a reactant in the silane hydrolysis and condensation reactions which result in polymerization.
In semiconductor processing, film curing is an essential step to allow the film to reach thermal equilibrium. Often times the thermal budget for back end of the line integration processing (i.e. metallization) is different than that of the temperature needs to form low dielectric constant (K) insulating layers, therefore it is essential that the low K film be exposed to the highest temperature it will see during integration processing to prevent remnant outgassing. Outgassing of the film will damage the device structures and decrease processing yield. Wafer curing via a high temperature hotplate could have beneficial effects in terms of improving the mechanical properties of the nanoporous silica thinfilm. A high temperature hotplate could increase the extent of crosslinking which would directly improve the mechanical integrity of the film. In addition to mechanical properties, high temperature hotplates could improve the wafer throughput due to the short processing times. Conventional SOG""s are typically cured via a horizontal tube furnace. The time required to reach thermal equilibrium is on the order of 30 minutes with a ramp up and a ramp down time period which more than doubles the over-all processing time.
In conventional wafer curing or drying of low K nanoporous dielectric films, SOGs routinely requires 30 minutes of elevated temperature exposure time. Ramp up and ramp down time for thermal equilibrium more than doubles the exposure time. Thermal equilibrium is needed to expose the films to the highest temperature during integration to prevent remnant outgassing. Wafer curing may have adverse effects on film hardness and mechanical strength.
The present invention solves these problems by conducting a series of processing steps which enable the production of nanoporous silica thin films with minimum process time and more consistent uniformity of film thickness.
The invention provides a process for producing a cured dielectric film on a substrate which comprises depositing a nanoporous silica dielectric precursor onto a substrate which precursor comprises at least one hydrolysed and condensed alkoxysilane composition, and then heating the substrate in a substantially oxygen free environment at a temperature of about 350xc2x0 C. or greater, for a time period of at least about 5 seconds.
The invention further provides a process for curing a dielectric film on a substrate which comprises the step of treating a suitable substrate comprising a dielectric film, in a substantially oxygen free environment by heating the substrate to a temperature of about 350xc2x0 C. or greater, for a time period of at least about 5 seconds.
The invention also provides a process for curing a nanoporous silica dielectric film on a substrate which comprises the step of treating a suitable substrate comprising a dried nanoporous silica film, in a substantially oxygen free environment by heating the substrate to a temperature of about 350xc2x0 C. or greater, for a time period of at least about 5 seconds.
The invention also provides a process for curing a nanoporous silica dielectric film on a substrate which comprises the steps of (a) suspending a suitable substrate within a heating element, in a substantially oxygen free environment, wherein the substrate remains free of contact with the heating element, the substrate comprising a dried nanoporous silica film; (b) curing the dried nanoporous silica film sufficiently to remove outgassing remnants from the dried nanoporous silica film; then, (c) removing the cured, dried suitable substrate from the heating element.
The invention further provides a process for curing a nanoporous silica dielectric film on a substrate which comprises the steps of (a) suspending a suitable substrate within a sealable hotplate, wherein the substrate remains free of contact with the hotplate, wherein the substrate comprises a dried nanoporous silica film; (b) sealing the hotplate, wherein the suspended substrate is contained therein; (c) passing an amount of inert gas across the substrate effective to displace non-inert gases adjacent to the substrate; (d) heating the hotplate to a temperature of from about 350xc2x0 C. to about 600xc2x0 C.; (e) contacting the substrate with the heated hotplate; and, (f) removing the substrate from the hotplate.
Additionally, the invention provides a process for curing a nanoporous silica dielectric film on a substrate which comprises the steps of (a) suspending a suitable substrate within a sealable hotplate, wherein the substrate remains free of contact with the hotplate, wherein the substrate comprises a dried nanoporous silica film; (b) sealing the hotplate, wherein the suspended substrate is contained therein; (c) drawing a vacuum within the hotplate effective to create a substantially oxygen free environment adjacent to the substrate; (d) heating the hotplate to a temperature of from about 350xc2x0 C. to about 600xc2x0 C.; (e) contacting the substrate with the heated hotplate; and, (f) removing the substrate from the hotplate.
By means of this invention, the dielectric constant of the nanoporous silica dielectric coating on a substrate achieves a low dielectric constant (K) below 3, while maintaining improved hardness and mechanical strength. Remnant outgassing is minimized with rapid curing periods that significantly increase film production rates.